Several applications depend upon accurate altimetry for performance. In aviation, aircraft separation, instrument flight paths and other navigation maintenance management functions are examples of such applications. This application discusses altimetry in the context of a specific application called ground proximity warnings, and more particularly, enhanced ground proximity warning systems (EGPWS). An EGPWS system is shown and described in U.S. Pat. No. 5,839,080 which is incorporated herein by reference.
Enhanced Ground Proximity Warning Systems require accurate geometric altitude referenced to mean sea level. To date, this input is provided by the aircraft's air data computer (ADC) in the form of Corrected Barometric Altitude. Barometric altimeters measure air pressure rather than geometric altitude. The conversion from pressure to altitude is based on an internationally agreed standard atmosphere (ISA). The ISA is most representative of average mid-latitude conditions. The ISA presumes that the atmosphere is static and dry, with air pressure dependent primarily on air temperature, gravity, and other physical constants. Atmospheric temperature is assumed to decrease at a constant rate up to 11 kilometers, the troposphere, at which point it is assumed to remain constant to 20 kilometers. Sea level temperature is assumed at 15 degrees C., and sea level pressure is assumed at 1013.25 millibars (mb). However, the real atmosphere can vary widely from this standard. This variation can cause errors in the indicated pressure altitude as compared with geometric altitude.
When we say "geometric altitude", we mean that altitude as defined by example in the treatise Introduction to Flight, 3.sup.rd Edition, (McGraw-Hill Series in Aeronautical and Aerospace Engineering, McGraw-Hill, November 1998) at page 70. in the example given in that treatise, if a helicopter hovering over Daytona Beach dropped a tape measure to the ground; "the measurement on the tape would be, by definition, the geometric altitude, i.e., the altitude above sea level." "Pressure altitude" is defined in the same treatise at page 79 as an altitude calculated using "the actual outside air pressure" (i.e., a local pressure measurement), as well as assumed (ISA) values for pressure at sea level, temperature at sea level, and temperature lapse rate (the assumed variation of temperature as a function of altitude). As discussed in the referenced treatise at pages 70 through 83, pressure altitude can be supplemented with correction factors for the actual pressure at sea level, the actual temperature at sea level, the local temperature, variations in the lapse rate from the assumed value, and variations in gravitational acceleration ("g"). Another expression for pressure altitude is "hydrostatic altitude", since the calculation is based on the known hydrostatic equation which calculates height of a column of gas as a function of the difference in the pressure of the gas between the top and bottom of the column. As used herein, "pressure altitude" shall mean altitude calculated using only a local pressure measurement and standard numbers, as well as that altitude and further including correction factors to account for variations from the standard (assumed) values. Unless specified otherwise, when the expression "altitude" is used herein, it is understood that we mean the estimation of geometric (or "true") altitude, using the described methods or apparatus.
The errors in the indicated pressure altitude mentioned above are typically not of concern at aircraft cruising altitudes since the primary use of the altimeter is to maintain separation between aircraft at different flight levels. Aircraft in the same general area experience the same atmospheric conditions, and therefore the same pressure altitude, within instrumentation limits. For instrument approach procedures, true geometric altitude is critical to maintaining safe terrain clearances. This is also the case for the EGPWS. Although the EGPWS design does make some allowance for altitude errors in its alerting envelopes, large errors in pressure altitude can cause the EGPWS to not give an alert when needed or alternately to give false warnings. Some atmospheric conditions which can lead to a non-ISA condition are listed below:
1. Sea level pressure non-ISA. PA1 2. Temperature warmer or colder than ISA PA1 3. Large vertical gusts due to, for example, extreme weather conditions, mountain waves, and other atmospheric phenomena. PA1 1. Pressure altitude calculation 103 from static pressure, and can include local temperature. PA1 2. GPS altitude processing 105. PA1 3. Calibration of pressure or other altitude with GPS altitude 107. PA1 4. Calibration of pressure or other altitude with radio altitude 109. PA1 5. Corrected barometric altitude processing 111. PA1 6. Overall blending and reasonableness checking 113.
Non-standard sea level pressure is normally accounted for by the pilot manually adjusting the aircraft's altimeter to the local pressure setting, i.e., corrected barometric altitude. Non ISA temperature errors can be quite significant. When the temperature is colder than ISA the indicated pressure altitude is higher than the true geometric altitude, when the temperature is warmer than ISA the indicated pressure altitude is lower than the true geometric altitude. Other temperature errors are caused by non-standard lapse rates due to inversions and other atmospheric conditions. Of particular concern are low level inversions which occur during the evening and in winter regions with snow cover.
Another source of error occurs when an aircraft travels horizontally across the earth's surface following a constant pressure surface. The use of corrected barometric altitude can introduce other errors as well. For example, since corrected barometric altitude relies on the pilot to enter the correct local pressure, this can also lead to errors. In another example, the use of QFE altitude settings, where the altimeter is adjusted to read zero on the runway, cause problems in the EGPWS.